Sounds like rocks in a can under my hood
 

I have a 30th anneversary 97 z28 camaro. when i went to start it this evening. in the rain (go figure). When i first started it there was a loud noise like a rock show out the block and it stalled out. i started it again this time it started fine. the engine was idling normal and it wasn't shaking sputtering anything. just the loud sound as if rocks were bouncing around in a can. it seems to be coming from underneath the block on the passengers side. since the engine seemed to be idling fine and i could accellerate without the sound altering in anyway. it would stop here and there but the sound is consistant. I decided to drive it up the street to try and get a better idea. the car accellerated fine. there was no hesitations, no backfires.. just the presence of this sound that seemed to not be noticeable as i got up to the posted speed limit of 40mph but as i slowed down to pull back in the driveway it became noticeable again. someone must know what's going on. im not very knowledgable about vehicles so i appologize if this is kind of vague. if anyone can help it would be much appreciated.

Havve you looked at the oil pressure?

if the noise does not change with engine speed then it would have to be a fan or air pump. can you make a recording of it and post it.

i have been watching the oil pressure and it is getting the same pressure as it usually does. today i took a look at it during the day and it's definately coming from underneath the vehicle. could it be an exhaust issue? i will try and record the way it sounds and post it.

could be exhaust, the cat will rattle when it goes to pieces.